They’re where the opportunities are. In 2014, the world’s 300 largest cities accounted for 20 percent of the world’s population and nearly half of global output. It is estimated that growing cities could bring nearly $30 trillion a year into the global economy by 2025.

As we rapidly urbanize—and 70 percent of urban growth takes place in emerging economies—understanding cities becomes critical. How can we, for example, improve livability and resource management? Manage disease and sanitation?